CN1390157A - Method and apparatus for particle agglomeration - Google Patents

Abstract

Fine particles of dust and other pollutants in gas streams are agglomerated to form larger particles which are more easily filtered in downstream processing. In one embodiment, particles in successive portions of the gas stream are charged with opposite polarity, and the gas stream is introduced into an Evase portion (12) to slow it down. Particles of different sizes have differential deceleration and therefore mix generally in the direction of flow, leading to agglomeration of oppositely-charged particles. In another embodiment, a gas stream is divided into substreams in respective parallel passages, and the particles in adjacent passages are charged to opposite polarity. Deflectors at the downstream end of the passages cause substreams of particles of opposite polarity to mix, with resultant agglomeration of oppositely charged particles.

Description

Method and apparatus for agglomerating particles

The present invention relates to methods and devices for agglomerating particles, in particular to electrostatic agglomerators used in air pollution control.

Background technique

Many industrial production processes emit small harmful particles into the atmosphere. For example, due to the presence of heavy metals and heavy organic material components in coal, toxic by-products of coal combustion that are suspended in the air tend to aggregate into scattered fine particle fractions. Many trace metals, such as arsenic, cadmium, nickel, selenium, and their compounds volatilize at high combustion temperatures, and then uniformly nucleate or condense on dust particles as the flue gas cools. The same goes for some toxic organic air pollutants.

Toxic particles formed by uniform nucleation are very fine submicron particles. These particles can enter people's respiratory system, so they pose a great risk to public health. The established link between toxicity and breathing comfort has prompted governments around the world to pass legislation to impose stricter controls on the emission of particles smaller than 10 microns in diameter (PM10), and especially smaller than 2.5 microns (PM2.5). Government regulations governing the emission of particles, especially fine particles between micron and submicron in size, will become more stringent in the future as the harmful effects of these particle emissions become more widely understood.

Smaller particles in atmospheric emissions are also a major contributor to the detrimental visibility effects of air pollution, for example, in coal combustion installations, the opacity of the flue is dominated by the fine particle fraction of the dust, because the extinction coefficient in light The maximum is reached near the wavelength of , which is between 0.1 and 1 micron.

The importance of fine particle control can be understood by considering the pollutant particles rather than the number of pollutants in the effluent. In the dust generated during a typical coal combustion process, pollutant particles with a size smaller than 2 microns may only account for 7% of the total pollutants, but they account for 97% of the total particle number. A process technology that removes all particles larger than 2 microns may be effective on the premise that it can remove 93% of pollutants, yet 97% of particles, including more respirable toxic particles, are still retained.

Various methods have been used to remove dust and pollutant particles from air streams. While these methods are generally good for removing larger particles from air streams, they are always very ineffective at removing smaller particles, especially PM2.5 particles.

It is known to use particle agglomeration techniques to agglomerate smaller particles into larger particles which can then be removed very easily and efficiently. Known agglomeration techniques include: (i) injection of chemicals into the air stream to increase agglomeration of fine particles; (ii) use of laminar flow precipitators to enhance surface agglomeration of fine particles; (iii) sonic perturbation suspended in gas (iv) applying an AC or DC electric field to perturb charged dust particles suspended in a gas to increase interparticle mixing, thereby increasing agglomeration; and (v ) bipolarly charges particles in a gas stream for electrostatic attraction.

Examples of known surface agglomeration techniques can be found in US Patent No. 5,707,428, and examples of AC electric field perturbation methods can be found in European Patent Application No. 0009857.

These techniques are often costly to implement in large installations, and chemical injection can cause other health concerns. Furthermore, these known techniques are not particularly effective for fine dust particles.

The most common agglomeration technique is surface agglomeration. In surface condensation techniques, particles must come into contact with a collecting surface or body to be removed from the gas stream. Large particles larger than about 10 microns in diameter can be captured relatively easily by inertial action devices such as impacts, interceptions, and centrifugal forces. In the electrostatic precipitator, because the large particles can carry more charges, they are subjected to a greater electric power, so they are collected very easily.

However, as particle size decreases, the mass of the particle decreases proportionally to the diameter of the cube, and inertial forces are poor at bringing these particles toward the collection surface. These small particles also have little charge, so they experience less electrostatic force. For particles smaller than 0.1 microns, diffusion is usually the main mechanism of action for particle transport, charging and trapping, however, for particles between 0.1 and 0.2 microns, diffusion, electrostatic and inertial effects are not very strong, known Devices using these principles of action exhibit the lowest collection efficiencies for particles in these size ranges.

The effectiveness of diffusion trapping can be improved by providing a large surface area and/or more time for diffusion to occur, but this requires a substantial increase in device size. Greater inertial forces can be obtained by increasing the relative velocity of the particles relative to the collection surface, but this is at the expense of greater pressure differential and energy input to the collection device, which results in a substantial increase in cost. Therefore, economical considerations limit the application of these methods.

Other dust collection devices used for fine particle emission control include wet electrostatic precipitators and wet precipitators, which generally require large and expensive equipment and create problems of polluting wastewater. Fiber screens have also been used as dust collectors, but they are generally ineffective collectors for fine particles because these small and often smooth particles tend to flow through the fabric used in such screens.

It is an object of the present invention to provide an improved method and apparatus for particle agglomeration.

Contents of the invention

In its broadest form, the invention provides an apparatus for agglomerating particles in a gas stream comprising:

an ionizer for oppositely charged particles in the gas stream, and

A structure placed downstream of the ionizer to physically redirect the flow of gas to allow mixing of oppositely charged particles, thereby promoting particle agglomeration.

Said in another form, the present invention provides a method of promoting agglomeration of small particles in a gas stream comprising the steps of;

Charge the particles in the gas stream with opposite polarity, and

Physically altering the direction of the gas stream to mix oppositely charged particles, thereby promoting particle agglomeration.

While the use of ionizers to charge particles in a gas stream is known, the prior art relies primarily on diffusion to bring the charged particles into proximity for electrostatic forces to be effective. As mentioned above, these techniques are often ineffective. The invention involves physically altering the direction of gas flow to facilitate mixing of oppositely charged particles.

The present invention can be implemented economically because it employs a relatively simple passive structure downstream of the ionizer to mix the oppositely charged particles and thus facilitate their agglomeration.

In one embodiment, an AC ionizer is used to charge particles of opposite polarity in successive portions of the gas flow. The AC ionizer may include a set of electrodes positioned transversely across the gas flow to which voltage pulses of alternating polarity are continuously applied. The electrode set may comprise a series of spaced apart elongate members having pointed protrusions on which the ion discharge occurs.

The structure may be an Evasé section or similar structure in which the cross-section of the gas flow is expanded, thus reducing the velocity of the gas flow. Due to the differential motion of the particles in the gas stream in the direction of the stream, particles of a single polarity from one portion of the gas stream will mix with particles of the opposite polarity from previous or subsequent portions of the gas stream. When oppositely charged particles are in close proximity, they are likely to attract each other and thus clump together.

In another embodiment, the ionizer is a bipolar DC ionizer which charges particles of opposite polarity in adjacent portions of the flow through the gas. The DC ionizer may comprise a plurality of sets of electrodes arranged transversely across the gas flow, each set of electrodes connected to a DC voltage such that adjacent sets of electrodes are opposite in polarity.

Each electrode set is positioned in the flow direction of the gas stream and may comprise a series of spaced apart elongate members having pointed protrusions. A plate assembly is disposed between and positioned parallel to the electrode sets. The plate assembly provides a grounded surface.

In a second embodiment, the structure may include gas flow direction deflectors downstream of the respective electrode sets for mixing adjacent portions of the gas stream containing oppositely charged particles passing through adjacent electrode sets.

The mixing effect of charged particles can be enhanced by acoustic perturbation downstream of the ionizer.

It is also possible to pre-treat the particles by spraying them with a compound, such as ammonia, to make them "stickier."

In order that the invention may be more fully understood and put into practice, embodiments of the invention will now be described with reference to the accompanying drawings.

A brief description of the drawings

Figure 1 is a schematic perspective view of a particle agglomerator according to one embodiment of the invention, employing an AC ionizer;

Figure 2 is a front view of the AC ionizer in Figure 1;

Figures 3(a) to 3(f) show alternative barbed electrode wires for the ionizer of Figure 2;

Figures 4(a) to 4(d) show the voltage waveforms applied to the AC ionizer in Figure 2;

Figure 5 is a schematic plan view of a particle aggregator according to a second embodiment of the present invention, employing a DC bipolar ionizer;

Figure 6 is a plan view of the bipolar ionizer in Figure 5;

Figure 7 is a front view of the bipolar ionizer in Figure 5;

Figure 8 is a partial perspective view of an electrode set of the bipolar ionizer of Figure 5, showing an air guide plate; and

Fig. 9 is a schematic plan view showing air flow around the air guide plate.

Description of preferred embodiments

1-3 show a first embodiment of the particle agglomeration device of the present invention. In this embodiment, pre-charged particles of different sizes in the air stream are given phase-differential velocities in order to enhance mixing of the particles in the longitudinal direction of motion. This increased mixing leads to agglomeration of the particles.

As shown in Figure 1, a pipe 10 with a substantially constant cross-section is connected to a second pipe 11, which has a substantially constant cross-section and is much larger than the cross-section of the pipe 10, which is passed through a cross-section A progressively larger Evasé portion 12 is connected to the pipe 11 . The ducts 10, 11, 12 provide a duct for the gas flow.

An AC ionizer 14, shown schematically in block form in FIG. 1 and shown in more detail in FIG. The AC ionizer 14 includes a series of spaced apart electrodes 15 resting between top and bottom busbars 16 . The top bus bars 16 are supported by the top of the duct 10 through insulators 17, while the bottom bus bars 16 are connected by insulators 18 to support rods 19 suspended from the top of the duct. The electrodes 15 are arranged in a vertical planar group fixed transversely through the passage of the pipe 10 . The electrodes 15 are connected through suitable voltage control circuitry (not shown) to a high voltage AC power source, preferably greater than 1KV, typically 20KV to 100KV.

The electrode 15 may suitably be a single or multi-strand wire, or be in the form of a mesh. Preferably, electrode 15 is a barbed wire or strip having points, spines or prongs along its length, an example of such an electrode is shown in FIG. 3 .

The electrode 15 can be manufactured from a flat strip with V-shaped thorns on one or both sides. These spines may be in the same plane as the flat strip, or twisted at an angle to improve ion generation and distribution. The spines or other points on the electrodes can be twisted or angled to direct the ions in a desired direction, and the spacing of the electrodes 15 can be varied to adjust the ion generating corona characteristics. The level of ionization typically depends on the number of spines or other points along the electrode. The ends of the electrodes 15 can be provided with springs so that the electrodes can be tensioned between the bus bars to keep them straight.

A high voltage applied to the electrodes creates a strong electric field around the tip, spine, or prong, and a corona discharge occurs. The discharge ions of the electrodes 15 attract particles in the passing gas stream, thereby charging them. While normal wire electrodes are capable of generating ions to charge the particles, using this type of barbed electrode results in better ion generation.

The high voltage AC power supply to the electrodes 15 is controlled by a controller employing a microprocessor which uses solid state power switches such as SCRs or IGBTs to regulate the voltage supplied to the electrodes. Properly adjusting this voltage maximizes ion generation without sparking or arcing.

In application, the first conduit 10 receives a relatively high velocity gas stream containing dust and/or other contaminant particles. An AC voltage as shown in FIG. 4(a) is turned on, so that pulses of opposite polarities are continuously applied to the electrode 15, and typical voltage pulse waveforms applied to the electrode 15 are shown in FIG. 3(b) and 3(c). For 50Hz AC, the polarity inversion occurs every 10ms. As shown in Figure 3(d), this period can be increased by skipping cycles, thereby reducing the frequency of polarity switching. Alternatively, the frequency of the AC power can be varied.

The polarity of the ions generated by the electrodes 15 alternates over time. Since the ions charge the particles in the passing gas stream, the gas stream passing over the AC ionizer will contain successive cross-sections of oppositely charged particles spaced apart in the direction of motion.

The piping structure downstream of the ionizer changes the flow characteristics of the gas flow, ie, when the gas flow enters the Evasé section 12, its volume will increase, thereby causing a corresponding decrease in the average velocity of the gas flow. Because the particles entering the gas stream are different sizes, they have different kinetic energies and momentums, and thus, larger particles will not decelerate as quickly as smaller particles. Due to their differing velocities, particles of different sizes will mix in the general direction of gas motion, that is, some particles of one polarity in one cross-section of the gas flow will be of a different polarity than in other cross-sections particles mixed. Because charged particles of opposite polarity are in close proximity to each other, they will attract each other and agglomerate into particles of larger size.

The gas stream is then sent to other dust collection devices, such as electrostatic precipitators or fiber filters, where the increased size of the particles enables these devices to collect dust more effectively. Agglomeration can also reduce health hazards by making dust particles larger and thus less likely to be inhaled into a person's respiratory system.

The gas flow fills the enlarged cross-section of the Evasé section 12 causing it to expand laterally. This lateral expansion also promotes the mixing of particles of different sizes in the gas flow. Smaller particles are likely to cross laterally across oppositely charged larger particles. movement path.

Agglomeration of particles can be enhanced by pretreatment. Suitable pretreatment methods include spraying the gas stream with ammonia, which increases the "stickiness" or viscosity of the dust particles and thus increases the bond strength between agglomerated particles.

The mixing of the particles in the Evasé section 12 can be further enhanced by acoustic disturbance using a A series of horns or vibrators 13 fixed on the Evasé part 12 .

To enhance particle charging and reduce particle agglomeration on the pipe walls, an electrically insulating coating is used on the inner faces of the high velocity pipe 10 and Evasé section 12, which will prevent ion consumption on the grounded metal pipe, thereby increasing the density of particles in the gas flow. Electrically isolating the duct 10 and the chamber of the Evasé section 12 also prevents charged dust particles from being electrically attracted to and adhering to grounded steel ductwork.

The AC ionizer may comprise an additional set of plates of electrodes 15 fixed through the passage of the pipe. Where several spaced electrode sets 15 are used, the period and shape of the AC voltage pulses applied to the electrode sets are controlled to optimally charge the particles and avoid charge cancellation with subsequent electrode sets.

A second embodiment of the invention is shown in FIGS. 5 to 9 . In this embodiment, a gas stream containing dust particles and other contaminants is divided into a series of parallel streams that are passed through a bipolar charger so that particles in adjacent streams are charged with opposite polarity. charge. The streams are then deflected so that adjacent streams merge and/or cross, thereby increasing mixing of the particles and enhancing agglomeration, that is, when streams merge or cross, charged particles of opposite polarity will come into close proximity and mutual attraction. They thus agglomerate into larger particles which can then be filtered from the gas stream very easily using known techniques.

As shown in Figure 5, duct 21 receives a high velocity gas stream containing dust particles and other contaminants in the direction of the arrows shown in the figure. Pipe 21 can be connected by an Evasé section to a larger pipe 22 with the purpose of reducing the gas flow rate for subsequent filtration or discharge. A bipolar ionizer and coalescer 24 is placed within the conduit 21 , more specifically shown in FIGS. 6 to 9 .

The bipolar ionizer 24 includes a series of parallel plate electrode sets 25 aligned with the gas flow direction and spaced across the conduit 21 . In the illustrated embodiment, the electrode sets 25 are vertical, but they could be horizontal or angled as desired. Each electrode group 25 comprises a series of spaced apart wires or flat strips extending between top and bottom busbars 26 which serve as support seats for the wires or strips. The electrode set can be constructed according to FIGS. 2 and 3 as described above. The electrode set may be of mesh, stranded wire or other suitable configuration rather than the barbed wire or straps shown in Figures 2 and 3 above to enhance ion generation.

Each electrode group 25 is fixed on the wall of the pipe 21 by an insulator 27 . The electrode groups in odd order are conductively connected by conductive strips 28 , while the electrode groups in even sequence are electrically conductively connected together by conductive strips 29 . In use, the conductive strips 28, 29 are respectively connected to the positive and negative output terminals of the high voltage DC power supply, so that adjacent sets of electrodes are charged with opposite polarities.

The DC power supply is preferably greater than 1KV, typically 20KV to 100KV. The DC power supply is suitably controlled by a controller employing a microprocessor using solid state power switches. The positive and negative voltages applied to the bipolar ionizer 24 are independently controlled to ensure maximum balanced ion generation without sparking.

A grounded plate assembly such as a screen grid or plate 30 may optionally be placed between electrode sets 25 as shown in FIG. 6 . Thus, the gas flow entering the bipolar ionizer will be divided into parallel sub-flows located between the grounded plates 30 (or between the grounded plates 30 and the grounded conduit wall). Ionization electrodes 25 are suspended in the center of each channel. Each shunt flows along both sides of the electrode set, being effectively separated longitudinally by the electrode set.

Particles generated by the electrode set 25 move towards the grounded surface and are attracted to passing dust particles suspended in the shunt, thereby charging the particles. The ionization electrodes 25 in channels of odd order have opposite polarity to the ionization electrodes in channels of even order, therefore, the dust particles in adjacent channels have opposite polarity.

The channel defined between the ground plates is typically between 200mm and 300mm wide and sufficiently long in the flow direction to ensure that dust particles become charged as they pass through the channel. In a typical installation, a 4m wide duct is formed with 10 parallel channels, each 400mm wide and 8m long.

A V-shaped guide plate 31 is provided at the end of each electrode group 25, as can be seen more clearly in FIG. 8 . The deflector creates turbulence at the downstream end of the bipolar ionizer to enhance mixing of charged particles. Adjacent charged particles of opposite polarity attract each other and stick together, resulting in agglomeration of the particles. In particular, the V-shaped deflector deflects the streams so that adjacent portions of adjacent streams of oppositely charged particles merge or intersect, as shown in FIG. 9 . This guiding structure enhances mixing of oppositely charged particles, thereby enhancing agglomeration.

Although only a V-shaped guide plate is shown, any shape that enables efficient mixing of oppositely charged particles may be used, such as flat transverse plates that enhance downstream turbulence and thus mixing.

The turbulent mixing downstream of the bipolar ionizer 24 can be further enhanced by sonic perturbation using a series of horns or vibrators 32 fixed to the pipe 21 (FIG. 4). Other suitable particle agitation devices may also be used.

As the gas passes through the Evasé section 23, its velocity decreases as the volume expands. Particles of different sizes are reduced in velocity at different rates, allowing further mixing and condensation of charged particles up the gas stream.

As described above with respect to the embodiment of Figures 1-3, an electrically insulating coating may be used on the inner faces of the conduit 21 and Evasé portion 23 to prevent ion depletion at the ground plane, thereby increasing the particle density in the gas. It is also possible to arrange a dust collection chamber below the ionization channel in the bipolar ionizer 24 to collect the dust falling from the channel wall.

As with the first embodiment, the gas fed into line 21 may be pre-treated with ammonia or other chemicals to increase the viscosity of the particles.

The foregoing describes only some embodiments of the invention, and changes may be made to the invention which will be apparent to those skilled in the art without departing from the scope of the invention as defined in the following claims. For example, the V-shaped guide plate 31 can be fixed on the tail end of the ground plate 30 instead of the end of the electrode group 25 . The deflectors may be positioned horizontally rather than vertically as shown.

Although V-shaped deflectors are depicted in the figures, other shapes or configurations can be used as deflectors, including horizontal grilles, profiled plates, and winged devices that create mixing vortices.

The winged device may be triangular in shape and angled to the direction of flow of the airflow to create a vortex at the tail end of the delta wing.

Several rows of guide plates can be arranged in a staggered structure to produce continuous guidance and mixing for split flow, resulting in comprehensive mixing of particles and more opportunities for particle agglomeration.

Further, the electrode group 25 may be energized intermittently rather than continuously.

Furthermore, tubular or honeycomb arrays can be used instead of parallel channels in bipolar ionizers, high-density ionizers that can generate bipolar ions to charge particles.

Throughout the specification and claims, as long as the context permits, the word "comprising" should be interpreted in the sense of containing itself to include the whole described, and not necessarily exclude other situations.

Claims (24)

1. be used for the device of condensed gas stream particle, comprise:

An ion generator is used for making the electric charge of opposite polarity on the particle band of gas stream; With

A structure that is positioned at the ion generator downstream is used for physics and changes the flow direction of gas stream so that the cohesion that the charged mix particles of opposite polarity also improves particle thus.

2. device as claimed in claim 1, wherein, ion generator is the AC electro-dissociator of opposite polarity electric charge on the particle band in the continuous part that makes gas stream.

3. device as claimed in claim 2, wherein, this AC electro-dissociator comprises that at least one is horizontally through electrode group and a circuit that is used for applying to this electrode group the alternating polarity potential pulse that gas stream location is provided with.

4. device as claimed in claim 3, wherein, the electrode group comprises that array of spaced has the elongated member of most advanced and sophisticated projection thereon.

5. device as claimed in claim 2, wherein, structure is an Evas é part, the cross-sectional area of gas stream enlarges in this Evas é part, reduces the flow velocity of gas stream thus.

6. device as claimed in claim 5 further comprises at least one sound wave disturbance device, is used for stirring the particle in the gas stream of Evas é part.

7. device as claimed in claim 5 comprises further that in Evas é part one or more physique bodies strengthen the mixing of particle to produce turbulent flow.

8. device as claimed in claim 1, wherein, ion generator is a bipolar DC electro-dissociator, is used for making the electric charge of opposite polarity on the particle band of the adjacent part that crosses gas stream.

9. device as claimed in claim 8, wherein, this DC electro-dissociator comprises many isolated electrode groups that gas stream is arranged setting that are horizontally through, and in use, each electrode group is connected on the dc voltage, and the polarity of adjacent electrode group is opposite.

10. device as claimed in claim 9, wherein, each electrode group location is arranged on the flow direction of gas stream, and this each electrode group comprises that array of spaced has the elongated member of most advanced and sophisticated projection thereon.

11. device as claimed in claim 9, the flat component that further comprises between the electrode group and be provided with electrode group positioned parallel, this flat component provides earthed surface.

12. device as claimed in claim 8, wherein, structure comprises that at least one is used to the gas flow guide plate that adjacent part is mixed.

13. device as claimed in claim 9, wherein, structure comprises one or more V-arrangement gas flow guide plates that are fixed on the downstream of each self-electrode group, is used to mix contain the adjacent part of gas stream of being filled with the particle of opposite polarity electric charge by the adjacent electrode group.

14. device as claimed in claim 8 further comprises at least one sound wave disturbance device, is used for stirring the particle from DC electro-dissociator gas stream downstream.

15. device as claimed in claim 8 further comprises one or more physical units that are positioned at the electro-dissociator downstream, is used for producing turbulent flow at gas stream.

16. a method that improves the small-particle cohesion in the gas stream comprises the steps:

Make the electric charge of opposite polarity on the particle band in the gas stream;

Physics changes the flow direction of gas stream so that the mix particles of band opposite polarity electric charge and the cohesion that improves particle thus.

17. a method as claimed in claim 16, wherein, the particle in the continuous part of gas stream is filled with the electric charge of opposite polarity by an AC electro-dissociator.

18. a method as claimed in claim 17, wherein, structure comprises an Evas é part, and the cross-sectional area of gas stream enlarges in this Evas é part, reduces the flow velocity of gas stream thus.

19. a method as claimed in claim 16 wherein, is crossed particle in the adjacent part of gas stream is filled with opposite polarity by a bipolar DC electro-dissociator electric charge.

20. a method as claimed in claim 19, wherein, the mobile plate that is directed to of gas stream changes, and this guide plate mixes adjacent part.

21. a method as claimed in claim 16 further comprises the step of the charged particle in the sound wave perturbation of gas stream.

22. a method as claimed in claim 16 further is included in to make before the charged step of particle with the particle of compound in gas stream and sprays step with the viscosity that strengthens particle.

23. a method as claimed in claim 22, wherein, compound is ammonia or amino-compound.

24. be used for improving the device that gas flows the cohesion of micron-scale and submicron-scale particle, comprise:

Be used for making the device of opposite polarity electric charge on the particle band of gas stream; With

The flow direction of the change gas in charging device downstream is so that be with the mix particles of opposite polarity electric charge also to improve thus the device of particle coacervation one-tenth than the cohesion of macroparticle.

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